The present invention relates to dispersant viscosity modifiers for use in lubricants, especially for reducing soot-induced viscosity increase in heavy duty diesel engine lubricants.
Heavy duty diesel vehicles may use exhaust gas recirculation (EGR) engines in efforts to reduce environmental emissions. Among the consequences of recirculating the exhaust gas through the engine are different soot structures and increased viscosity of the oil at lower soot levels, compared with engines without EGR. It is desirable that oil exhibit minimal viscosity increase, e.g., less than 12 mm2/sec (cSt) at 100° C. at a soot loading of 6 weight percent.
It is also desirable that a lubricating oil composition maintain a relatively stable viscosity over a wide range of temperatures. Viscosity improvers are often used to reduce the extent of the decrease in viscosity as the temperature is raised or to reduce the extent of the increase in viscosity as the temperature is lowered, or both. Thus, a viscosity improver ameliorates the change of viscosity of an oil containing it with changes in temperature. The fluidity characteristics of the oil are improved.
Traditional dispersant viscosity modifiers (DVMs) made from ethylene-propylene copolymers that have been radically grafted with maleic anhydride and reacted with various amines have shown desirable performance to prevent oil thickening in diesel engines.
However, these materials can provide poor antiwear protection, especially in high soot conditions, leading to increased wear in the engine, for example on the valve train and/or various parts of the crankcase.
There is an ongoing need for dispersant viscosity modifiers that provide viscosity control but which also provide good wear protection.
The present invention, therefore, solves the problem of providing a dispersant viscosity modifier having improved performance in engine tests, providing a good viscosity index, providing good soot dispersion and/or toleration properties, while also providing good antiwear protection and/or film thickness performance, particularly in diesel engines, and especially in heavy duty diesel engines employing exhaust gas recirculation.